Trying to understand how the high speed idle is supposed to work

[valveamp]

I just installed a new (rebuilt) Autolite 4100 on my '71 Bronco. I broke (my fault) the base of the previous 4100 I had. Expensive mistake. Any way, it's basically running good now, but I've always struggled with the high speed idle operation. I THINK it supposed to work this way.....I set the choke before start up by pushing on the gas pedal. This seems to work correctly. Next, I start the engine and it starts up at a fast idle. In my case, fast is around 1500 rpm, or so. Hot idle is around 900 for me. As the engine warms, the choke slowly comes off (opens up). This seems to work ok. The problem is that if I tap on the gas, just after starting the engine, the idle goes back down to the normal "hot" idle, around 900. I thought that it was supposed to stay at high idle, then drop down to an "intermediate" idle, after a few minutes, then drop down to normal hot idle after a few more minutes. Is this correct? If so, what is not adjusted correctly on my choke that makes it drop to normal idle right after start and stepping on the gas pedal? I know there is a fast adjust idle speed screw under the choke coil and I have adjusted it, which is why I think the engine starts in high idle. I just don't know why it comes off high idle so fast. Appreciate any inputs or suggestions.

 

[jagbucket]

the black choke cap needs to turn to rich . think its a cc direction but the caps are marked so check it out . only move a small amount each time and test when cold again

 

[blubuckaroo]

It sounds like yours is working properly. I don't think there's an intermediate idle speed.
Is yours an auto tranny? If not, that idle speed is quite fast.

 

[valveamp]

A couple responses, first, I have the stock 3 speed tranny, no automatic. Second, I think I can adjust the fast idle speed down with the screw if 1500 is too high, but since it won't stay at 1500 after I just touch the pedal, it's not been any kind of a problem (being too high). It really doesn't matter that their isn't an intermediate idle speed (although the 4100 manual I have seems to suggest different idle speed plateaus), the real problem is the idle dropping down just after start with a touch of the pedal. Jagbucket, are you saying that if I adjust the choke in the rich direction, it will help with the problem of the fast idle coming down too fast? Just want to clarify. Thanks.

 

[75MIKE]

Not to answer for him, but, yes turning your choke to a little more rich will delay the choke coming off. I have to ask though, why are touching your gas pedal while on high idle right after you start it? That's the whole purpose of the high idle cam to let it warm a few minutes before you drive it. You want to be able to kick the idle down by stabbing the gas pedal. Not good to drop in to drive at 1500 RPM's.

 

[MarsChariot]

This happens when you have not set the choke and pull-off to factory specs. Fortunately, unlike many other carburetors out there, there are factory instructions for the actual real-world set-up of said carburetor. You need to read these two pages and follow the instructions EXACTLY. ( The "tool" in this case is a paper intermediate size clip. Not one of those big honkin' ones, nor one of those dinky ones.) Use the sequence numbered by hand on the margins of the paragraphs. Some Mustang savant discovered a few year ago that the instructions from the factory were out of sequence with the result that for many decades no one could set the carburetors right following the factory instructions. Follow the sequence and the 2100-4100 works just like it left the factory. These instructions are actual useful information. Not just fluff for some newbie. Follow them exactly. They work. I had exactly the same problem you are describing after some yahoo touched my carburetor. IT would not intermediate fast idle after touching the pedal. Now it does because I followed these instructions to the "T".
P.S. For the first attempt use a 1/8" drill bit for the clearance setting. If that does not work correctly, then go a bit size larger and then smaller on choke closure until it works. If still still not quite right do the same for the choke pull-off setting.

If you have any questions or don't have the "feel" for what the instructions are saying,please post here immediately for clarification.

 

[valveamp]

First off, let me say a big thanks to MarsChariot for posting these instructions. I'll go through them tomorrow and see what I can do. One question I have is do I need to remove the carb in order to perform the setup or can I do it while the carb is still installed? The answer may be obvious after I read the instructions. I guess is that it will be easier to perform the setup when the carb is off the Bronco.
To respond to 75Mike, I live in So. Calif, where it is almost always warm, so I don't have the need for the warm up like others may have. But, also, I have some other cars that use a carb with an electric choke and they will run at the higher idle even after you tough the pedal, until the choke has had some time to warm up and move. It was always my understanding that the purpose of the choke and high speed idle was to allow you to not have to wait and that you could drive just after starting without the engine stalling because it's cold. Maybe I'm wrong, but that's always how I understood it was suppose to work. Again, thanks for the information.

 

[MarsChariot]

valveamp, you can do the set up either on or off the engine. But the first time is easier with it on the bench. More comfortable and you can fiddle and see exactly how things work a bit better. After that you can do it on the vehicle. Understanding choke operation is something that requires wrapping your mind around how the choke restriction interacts with the idle transfer slot, main metering jets, and idle feeds. Too much choke and fast idle can cause the air speed to be too high and it pulls too much fuel. Too little and it gets too much air and goes lean until the intake warms up and starts vaporizing all that fuel. Hard to describe in a few words. Anyway, it is beginning to be a lost art and the more of us that learn how it works, the less chance that the knowledge will be lost to history.
Good luck and let us know how it goes with your next session.

 

[ared77]

MarsChariot, thanks for this. I'd imagine at some point I'll be setting up a 4100 on a Mustang, Bronco or Fairlane and this (good) info will be great to have!

Norm

 

[valveamp]

MarsChariot, I read over the instructions last night and I agree that doing it on the bench would be the easiest thing, especially for me. The only part that seems a bit confusing to me is where it describes how to insert the gauge tool into the choke "piston" tube (I think that's what it was called). I assume it will make more sense to me after I get it on the bench and have a look. Meanwhile gotta go find my paper clip......I mean gauge.. Carbs are a bit of a mystery to me on exactly how they work. It's fairly simple to understand at a high level, but when you get into the details it seems very complicated and I think that's what you are saying also. Thanks again.

 

[MarsChariot]

These instructions may be a bit clearer. From your description, the problem is almost certainly an improperly set choke pull-down clearance.

 

[Slowleak]

There are some YouTube videos that might help.... or cause confusion. This one is pretty good.
https://m.youtube.com/watch?v=Bve12T7zeC4

 

[valveamp]

I'm always amazed how much information is out there and that someone on this site has it and shares it. Yes, the additional instruction sheets are very helpful. Also, I looked at the video, but I'm in a room with other people, so the sound was down, but it looks to be great. A lot of detail on showing where everything is. I'll watch it tomorrow when I can turn up the sound. Again, thanks guys for the help.